Our earlier findings, that the incorporation of H3-thymidine into the DNA of synchronous CHO cells is a multiphasic function through the S-phase of the cell cycle, has attracted the attention of other researchers, notably Klevecz and coworkers. We shall attempt to determine whether the X-ray sensitivity of CHO cells mimics one, or more, of the labelled peaks. The studies of strand-break rejoining following X-irradiation of CHO cells sensitized with BUdR will be extended to discrete phases of the cell cycle, both for completeness and in order to ascertain whether experiments with asynchronous populations obscured a critical cell cycle dependence. We will continue to probe the very difficult area of the rejoining of DNA strand breaks and the reconstitution of the chromosomal DNA structure following low doses of X-rays.